Heat-absorbing window



Patented May 1, 1934 r PATENT o-*Fca HEAT-ABSORBING WINDOW walter Linke, Berkeley, Calif., dedicated to the People and the Government of the United States of America 'Application November 10, 1930, Serial No. 494,'707- In Great Britain November-21, 1929 V 5 Claims.

This invention relates to means for eliminating the substantially dark heat from the whole radiation of a light source, but it difiers from similar inventions already by its specialized object which consists in providing improvedand less expensive window-panes destined for absorbing the dark heat specifically of the sun, (which is a body of a temperature unequalled by that of any terrestial light source), and for 'transmitting as far as possible the light in amounts gradually increasing towa rds those rays of the spectrum for which the eye attains its maximum sensitiveness specically at daylight intensites of illumination. While prior inventions relating particularly to window-panes only, for instance that of the British Patent 197500, are quite suitable to fulfill the analogous purpose for the rays of artificial light, the panes of the present invention represent in fact an entirely 20 different article of manufacture, which would be inappropriate as a beat-filter in film projectors.

The invention comprises in the making of transparent` or highly translucent beat-absorbing window-panes mainly the selection and controlled employment of those compounds of the Chemical elements with atomc weights from 51 to which are showing absorption bands in the vicinity of the one or the other end, and especially beyond the red end, of the Visible spectrum,

and which have arelatively high transmissivity for the already mentioned luminous rays of the spectrum.

Reference is made to the drawing, wherein Figs. 1 to 4 represent various forms in which the invention may be employed, while Figs. 5 and 6 are explanatry diagrams.

Fig. 1 which needs no further description, shows the cross-section of a window-sash containing a solid pane in which the heat-absorbing compound is equally distributed, quite analogously to a pane made from any colored glass.

Fig. 2 representing the preferred form of the invention, shows a window-sash containing a pane of ordinary colorless glass, to 'the surface of whichthe heat-absorbing substance isapplied in the form of a preerably thin layer, indicated in the drawing by ablack lining.

Fig. 3 shows a sash containing a hoat-absorbing pane of the kind of Fig. 1, indicated by the n same manner of hatching, and

Fig. e a sash containing a pane or" the kind of Figure 2, behind which in each of these cases a second pane of ordinary glass is arranged. 'The light is intended to come in all of these figures from the leit side.

Fig. shows diagrammatically the difference between the spectral color luminosity of -objects in bright daylight (a) and in poor daylight or under the average artificial illumination of interiors (b), and

Fig. 6 the spectral difference between the radlation of the sun (c) and of an artificial light source' (d). Reference to these two figures' will be made farther below in this'description.

If the heat-absorbing compound is applied as a thin layer to `the outside surface of the pane, as exemplified by Figure 2, most of the absorbed energy is communicated by conduction and convection to the outside air, or is immediately reradiate'd through the blue sky into the-cold celes:

tial space.

Examples of performing the invention with the aid of a thin layer consisting of, or containing a material which fulfills the other requirements of the invention, are: glass fiashed at its original manufacture with a thin layer of such' material in a vitrified condition; or glass coated on the outside with any varnish or lacquer or lac containing such material or substance in true solution or as a stain, or containing it in the form of pcwdered glass or other Vitreous or crystallic material, preferably of approximately the same refractive index as the coating; or a thin sheet or film of the specified properties glued to the glass-pane or astened merely in contact therewith in the same frame. The absorbing layer may be applied by means of the transfer paper process, and, if composed according to ceramic principles, may be fused as a transparent glaze to the pane by firingafterwards. Two or more' layers with properties supplementing each other may be applied and fired simultaneously, especially if containing fiuxes with melting points gradually ascending in such manner that the layer next to the pane melts first.

,Artistic designs, applied by the said transfer paper process, or by hand by the glass painter, containing mainly coloring matters selected according to the principles of this invention, are within 'its scope.

suitable compounds of the afore-mentioned group of chemical elements are found many among those of iron, copper, and nickel The maximum of the most characteristic absorption band produced by iron' is Situated always around %3.001 or (well millimeter wavelength, and that or copper at 0.0%85 millimeter (or 8.85 in the near mira-red. Niclel has in a large number oi' its compounds a band at 0.0007 in the red, and another band found in nickel glass at '013022 seems also to be caused by this element. Cobalt compounds in true solution will probably show in the infra-red the broad but shallow absorption band known from cobalt glass. Nickel and cobalt oxide in glass, of course, are of no use for the present invention, because their transmission is exceedingly low in the central part of the visible spectrum, except perhaps in the case o! nickel oxide in strontian glass.

A plurality of absorbers with absorption-bands at different places in the inira-red, for'instance the combination of an iron and a Copper comof the present invention is that the light be as' much as possible balanced, as it were, in order to give good and non-atiguing vision. This is accoinplished by not only making the windowpane yellow-green but by also controlling its light absorption in such manner that the curve of the energy contents of the transmitted light elopes down gently and symmetrically to about zero orany desired value at both ends of the visible spectrum. The balancing color by which it is intended to suppress partially the usually too abundant blue and violet rays must iulfill, of course, just as well as the original color, the still more important condition-of a high transmission around 00005555 mm. wavelength.

The selection of the balancing color depends, oi' course, entirely on the'transmission curve of the heat-absorbers used. Uranium oxide and the various' means for producing a yellow or light amber color in glass may be mentioned here as probably useful. A great many spirit and oil soluble dyes of yellcwish or light brownish color will be useful in various varnishes and lacquers. Only very small quantities of these balancing colors are necessary in most cases, and a combination of two or three dyes in varying quantities will be a very pliable means to enforce any .desired absorption. The spectroscopeis the best means of dlsclosing the composition of the color, but n the case of the peculiar hue preferred for this invention a perfectly balanced transmission' extendlng over the (entire visible spectrum will also be indicated by the absence of dichromatism,

i. e. absence of a shifting of the dominant hue with the concentration of the absorber or the thlckness of the layer. Practical experiments'in most cases will lead more easily to the desired result than predetermining the absorption with the aid of known transmission values.

Chromium and vanadium oxide in glass are known to suppress to a certain extent both ends of the spectrum simultaneously, though the blue end considerably stronger. Many Organic dyes o! green color will fuliill the same purpose in varnish and lacquer coatings, though generally l'reey transmitting the infra-red and also al- A combinationof two different' dyes absorbing both ends oi the spectrum separately will have the same or even a better efiect, off course.- t the' substances usedfor suppressing the blue end contain iron, they can be presumed (with the caution which is always necessary' where conclusions !rom the color are drawn, or whe'rethe existence of a certain 'absorption band is generalized) to be heat-absorbing in spite of a yellow transmisslon color and in spite of having eventually their highest 'transmission in the extreme red.

If the absorption of the balancing color extends sufliciently' far into the ultra-violet, and if it is embodied in a separate varnish coat outside oyer the other parts of the filter, this coat will protect the rest of the filter against the weather and against the actinic rays ofthe sun. such protection is advisable in any case for coatings made from celluloid solutions.

More definite advices for performng the invention are given in the following.

A manufacturer of colored glass has in' all probability no other'choice but to produce an iron and copper hearing glass by prolonged heating under oxidizing` conditions. Many other means besides lead and boron are at disposal to Shift the dominant hue of the glass more into the yellow-green (for instance sodium in iron silicates and in copper lead-silicates, further Zinc and aluminum 'in certain cases), but no other 'eminent heat-absorbers besides the 'oxides of iron and Copper (and possibly vanadium oxide) in true solution in glass seem to exist for use with the present invention. g

A glass painter, whose products are intended to be fired under oxidizing conditions, may be advised to use a suitably selected modification of the usual transparent glazes containing copper oxide. Boric acid should be employed in quantity justsufficient to secure a good transparency. The shifting of the hue into the yellow green may be done by uranium oxide or better as much as possible by a yellow iron borate.

Very flnely powdered glass in which iron oxide and copper oxide is contained in true solution, is the only heat-absrber I wish to recommend at` present deflnitely for use in various kinds of varnish coats. If for some reason the powder, or a combination of two powders, for instance a copper silicate and a yellow iron silicate or' high soda contents, is not applied simply in mixture with the varnish, one could proceed by dusting it ona wet coat of varnish, the procedure to be repeated eventually onceor twice after drying. As in most instances the indices of refraction of glass and varnish are not very different, a semitransparent coat with low difiuse reflection will be obtained. By mixing a varnish of lower refractive index than glass with one of higher refractive index it is possible to produce a varnish the index of which is equal after drying to that of the powdered heat-absorbing glass. Eventually the refractive index of the glass may be predetermined in a similar manner by varying the composition of the glass. The knowledge of the following figures, representing the indices of refraction, will be useful for this purpose: Copal resin and, Canada balsam about 1.53, mastic 1.54, amber and colophony (rosin) about 1.545, Peru balsam 1.6, aloe resin 1.62, piperine (an alkaloid which can be fused together with colophony and other resins) 1.68, borax and boric acid (fused) about 1.463, fused silica 1.4585, ordinary glass 'ready the extreme red end of the 'visible spectrum.

quantitiesof iron'or copper oxides will prob-' ably increase the refractive index of ordinary ,siass quite considerably; Amber; hard copal, and the hands of an experienced man water p ton'` :true solution,

reducing conditions,` for instance Crooke's: sage most recommendable vetheoretically better to have the h ar-abparticles which is equivalent rather than relatively large 3 particles, 'for instance of in a. suspension of distributed in'a vehicle, because then the indices cases the' additional substance will be of refractio'n of the absorber and of the vehicle need not be equal. In order to attain this, it is necessary, in the first instance, that the absorber be soluble in one of the constituents of the varnish or lacquer. If "this is not the case, an attempt must be made to find a substance which is for the absorber and the sub- In many a volatile liquid which mixes with the solvent of the varnish or lacquer in the manner of a true solution, as or instance water and alcohol, but which does not -behave as water and oil with respect to one another. Ethyl acetate, for instance, is known to play this part in a great many cases. Further, duringthe evaporation of the solvent the absorber must not crystallize with such size of grain that the transparency'(or high translucency) is impairedt This necessity makes it a'common solvent stance forming the finished sheet.

probable that mainlydyes of high staining poweracid, nickel oxalate, and

'their smaller inclination to crysta'llize, as dyes .to 'be used in an evaporating solvent are for the are suitable for producing transparent lacquers,

and that the more complicatedorganic compounds of the mentioned metals, on account` of' present invention more suitable than the chetnically simpler ones. Cuprammonium may be mentioned as a heat-absorber which also is a solvent, (for cellulose). As example of heat-absorbers which the dyer is able to produce in a finishedlacquer coat as a stain, may be mentioned. the various copper compounds of acetic 'ferric ferro-Cyanide.

since there exist more than ahundred varieti lacs, and similar substances to be usei ifi commerciarvarnishes, and since it is known that the successful handling of every one of` them requires some experience, I do not wish to make. any definite suggestions for coloring these substances directly, (or without primarily coloring their solvents), except by the described admixture of heat-absorbing glass powder of approximately the same refractive index. However, there can be no doubt that without further invention many so-c'alled resinates and oleates with heat-absorbing properties are obtainable, or areknown to be obtainable, by fusing (or boiling) metal compounds with the varnish substances, or by precipitating theresinates in the known wet process. In the products variable quantities of metal oxides aredissolved or compounded in a transparent state in the meaning of this invention, and their analogy with the socalled silicates, borates, etc., is obvious. While copper oleates seem to have generally an intensive green color, the reslnates and oleates of iron are generally of a yellowish or brownish color, so that they can be used for suppressing the violet end of the spectrum, besides in all probability' contributing to the heat-absorption.

I do not claim any articles of manufacture showing an absorption in character quite different from the typical bands which appear when light is transmitted through the interier of com. pounds of iron, or copper, or nickel, and particularly if containing oxygen. This concerns especially copper and iron glasses produced under heat-absorbing glass,

floldal gathering of I i Glasses poor in oxygen and rich in iron particles, which in this case are tain the iron as a invention,

green (bluish green) glass or the heat-absorbing window-glassof 'the British Patent specification Small quantities of iron in a glass poor in oxy-- 'gen and especially after. quick chilling do not show a color corresponding to Beer's proportionality law of absorption, but are practically colorless, just as gold or copper glasses under the same conditions.

All glasses poor in oxygen must contain the iron necessarily in the elemental metallic state, because the process of their production is identical with the process of reducing iron ore. Small iron contents of one or only a few percent of the metallic element seem to give the best blue color. The selective absorption caused by colloidally Suspended metal particles in glass is in character very much different from that caused by substances in true solution, and consists mainly of extended regions of more or less uniformly low transmission, which in certain cases is nearly the exact reversal, 'or the complementary color, of the absorption of thin metallic films of the same substance. Tin, zine, and cadmium,` which all aremetals of high soldering and alloying power, have become known as promoting the blue colthe iron molecules.

probably much larger than in the former cases, are smoky grey or black. and especially in the letter case appreciably magnetic.

Only glasses rich in oxygen are able to conproper glass constituent. Their color is usually green or 'yellow-green, but may become displaced, bythe influence of boric acid or other glass constituents, towards the yellow or even into the red.` It isthis green or yellow green form, of oxidized iron glass which is quite exclusively used as an aborber'or the present as 'far as iron in :glass concerned, and which alone'show s a' hightransmissivity for yellow green light in combination with an infrared absorption band 'of suflicient depth and broadness. 'High and prolonged heating under oxidizing conditions, is necessary to form a proper iron silicate. By the 'oxidation the glass of this invention becomes in substance as different from theprior heat-absorbing glasses of blue and bluegreen color as oxidized green copper glass is different from reduced red copper glass made from the identical batch, though in the case of iron glasses the difference in color is not so striking.

Ferric oxide (FeOs, polishing rouge, crocus) shows already in the dry state differences of color which are caused merely by the size of the grains of the particular variety. The same is the case when the oxide is distributed in the glass undissolved or only partially dissolved, and able glass painters understand how to produce a large variety of rather different colors merely by the proper selection of the oxide and the flux and by controlling the time and temperature of heating. The hard ferric oxide seems to dissolve less easily than other iron compounds, and a fused soda or other fiux seems to be much less suitable for its dissolution than' aglass such non-metallic elements as arsenic, chlorine, selenium; which are of electro-negative character, or of negative valency,` have a strong tendency to collect the iron and other' coloring elements in statu nascendh layer, ,an additional in larger particles i greenish. yellow heat-absorption 4 !rom the less stable compounds and especially from the already existing cclloidal particles and. upon vaporizing, to take them entirely out oi the glass into the chimney; While the freedom' ot the original batch from "decolorizing" materials or impurities oi such electro-negative character appears to be necessary in order to keep the coloring materials in the mixture until after melting the respective colored silicates are iormed, the use oi arsenic in large pieces 'thrown into the molten glass, where it sinks to the bottom and quickly vaporizes, will remove generally only the remaining undesired colloidal impuritiea However, by too much of it also a considerable part of the colored silicates used in this invention. among which the true iron silicate is probably the most stable, may be destroyed again.

The color of glass containing the ferric oxide of still entirely ultra-microscopic size can be presumed to be in character similar to that of polishing rouge. This means that the high transmissivity at the extreme red end oi the visible spectrum continues into the mira-red to at least 00044 mm. wavelength, so that a glass containing the iron solely in the colloidal erric state would be heat-transmitting for every practical purpose.

Iron glasses of amber color, in which reduced iron is accompanied by rednced carbon or sulphur, or oxidized iron by. oxidized manganese, are not considered to be heat-absorbers in the meaning of this invention, because it is the very nature of any ambel' color that it has its highest transmissivity in the red andshows a continuous or evengradual decrease' towards the violet; This is irreconcilable with the 'principle of this invention, of providing in the yellow-green a transmissivity as high as possible, accompanied by a low transmission in the red and beyond. Only. if the amber color is very light, it may be used occasionally for subduing the abundantly transmitted by most copper glasses, though not only an easily procurable yellow or iron glass with a pronounced y but even any light yellow siass (which in contrast to the amber glass has an eq'ally high transmission for all long waves, including the yellow-green) would be in any case more adequate for this purpose.

A smaller or larger part of the iron is probably in every glass unintentionally or unavoidably present in the form of dissolved ferric oxide, or of a so-called iron silicate. As conipared with this uncontrolled occurrence of dissolved ferric oxide the present invention comprises the use of glass in which the ferric oxide is contained predominantly 'in true solution, and lntentionally, as a regular manuacture.

sheets consisting of relatively opaque and lightdiilusing material are not within the scope of this invention, regardless of whether heat-absorbing or not. A reflecting layer may be, combined, 'however, in a novel manner with the invention by applylng on the outside over the transparent heat-absorbing layer a semi-opaque white coat which partially reflects and partially transmits the light in a ,difiused manner. The reflected .part of the light does not intrude in this case the clear absorbing layer at all, while, when the white reflecting layer is applied behind the absorbing amount oi! radiation is abthe undesired through the absorbing layer.

so that the pane becomes consider-amy hotter than tlan, is also within in the first case, or even than without any re ilecting layer.

The intermediate case, in which a white (including infra-redreflecting) pigment is embedded in the' clear heat-absorbing layer of my inventhe scope of my generic claims.

Water soluble-coatings, and water soluble absorbers therein. while not objectionable as filters for artificial light sources in certain cases where n'o exposure-to' the weather or to the touch of human hands is intended, are deemed to be unsuitable for the present invention.

The improvements constituting my invention v are not inten ed to go beyond the limits of physical and physiological optics including those of invisible radia ioin in the neighbouring parte of the visible spectrum, and, as I have not had, till now, any occasion to work the invention out practically, my advice as that oi a laynan in chemistry and in the art of the dyer, cannot be inallible, and, therefore, is intended only to indicate the direction in which other hoat-absorbing materials, besides heat-absorbing silicates and the like, and processes for producing the same are atthe d'asposal of any expert in chemistry.

The following physical and physiological data. v and the conclusions drawn therefrom, not only form to a large part the foundation of my invention, but also indicate the preferred manner oi its performance. l l

- One and the same amor-.nt of radiant energy in the form of good or average daylight illumination by windows, *producing within the room by absorption in every case the same amount of hoat, will incite in the average eye the maximum amount of luminous sensat= on when consisting of light o0.0005555mm. wavelength (yellow-green). Light of 0000533 and 0000510 mm. (near both ends of the yellow-green region) will incite only 90 percent thereof, light bi 0.000510 (at the border between yellowish and bluish green) and 0.000614 (orange-yellow) only 50 percent, and light o! 0000475 and 0000652 mm. 'only 10 percent of the mentioned maximum sensation. These wavelengths are so chosen that the curve of luminous eiciency of good daylight may be easilycompleted on paper by making it practically zero at 00004 and 0.00072 mm., though it extends tor most persons in minute traces at least until 0000397 and 0.000760 mm., where the H and A lines of the solar spectrum were discovered visually. These figures are valid only for at least the illumination intensity of daylight interiors, which can be taken roughly as 10 millilambert (one millilambert being the thousandth part of a lambert, or of the brightness of a perfectly whiteand 'perfectly diiiusing surface ill'minated by the standard sunshine of 10000 meter-candles) while the average illumination of night interiors is roughly 0.1 millilambert. Below 10 millilambert, and especially within the relatively narrow range between 1 and 0.1 millilambert, however, the maximum sensitiveness of the eye shifts rapidly from 00005555 to 0.000510 mm. wavelength', but seems to move thereaiter even for very much lower intensities only slightly farther. The last named figure, on account of its smaller importance and of the more complicated conditions ci research has been ascertained only with incomparably less accuracy than the figure for good daylight illumination.

There seem to exist inner reasons why this range is ldentical with that unique region of the spectrum for which no' single complementary color exists, because it is not green enough to 150 55 above the absolute zero of temperature.

seems for that illumination intensity, for which this hue functions as maximum of luminous efiiciency, to be the only one which allows such spectral distribution of its energy that the corresponding luminosity curve declines symmetrically unto the extreme limits of visibility in the red' and violet, and which then will not show dichromatism, or a shifting of its hue, with increasing 'or decreasing saturation of the coloring agent. Any other color could be made free from dichromatism only by suppressing a more or less considerable region at one of the ends of the spectrum entirely. I

Fig. 5 shows the luminosity curves of the best possible (a) and of that poorest daylight illumination (b) at which a change to artificial illumination should be made. While the wavelength of 0.0005555 mm., or 0.556, which is marked in this figure as the maximum of one of these two curves, is the primarily preferred hue for this invention,

5 with any greenish yellow hue included as a less efiicient equivalent, the wavelength of 0.000510, or 0.510u, which is marked in the figure as the maximum of the other curve, and which separates the yellowish green from the bluish green,

is deemed to be the natural border line of the invention towards the short-waves.

As a standard for the color work involved in the present invention I recommend three solutions of 57.52 grams of cupric chloride, 1.22 grams of potassium'bichromate, and 8.22 grams of ferric 'chloride, respectively, dissolved in' each case in 1000 cubic centimeters of distilled water, and filled in three successive cells of colorless material, each of 1 centin'eter inner width. If instead oi' these 49 cells rectangular bottles of, for instance, 1.5 centimeters width would be used, the solutions would have to be diluted proportionally, using in this case 1500 ccm. of` water instead of .1000. This threefold filter, as originated by Karrer :for photometric `work h as a curve of spectral transmission which very closely approaches the luminosity curve of radiation as found already with ample exactitude by the earlier work of Ives, with a maximum 'around a wavelength oi 0.00055 mm. or

50 slightly more.

Fig. 6 illustrates the difference between the radiations of the sun (c) and an artificial light source (d), or more exactly theradiations of two perfect black bodies of 6000 and 2000 centigraids e areas under the two curves are intended to be roughly equal, thus representing equal energy or equal heat. It is already from this figure apparent that for the two light sources different I e o requirements will have to be fullled by filters for 'the same, and the nature of these requirements is set forth more clearly in the following.

The distribution of energy in the solar spectrum was found in the city of Washington on the average as follows, when the sun was 30 degres above the horizon (arbitrary units).-; 130 at 0.0004 mm. in the violet, 323 at 0.0005, 306 at 0.0006, 268at 0.0007 in the red, 141 at 0.001, '55 at 0.0015, and 21 at 0.002'mm. For the sun standing in 'the zenith' and for high altitudes /above sea' level the violet and ultra-violet component of the 'energy curve increases very considerably, but the near-infrared Component only very slightly. Further, in the regions near the 7 equator the air contains according to Hann on the average 2.63 percent water vapor, at 50 degrees northern latitude 0.92 percent, and at '10 degrees only .0.22 percent. It. is well known that water vapor begins to absorb at 0.0015'mm. in the 'infra-red, and that the presence of large quantities q! it allows scarcely any transmission beyond 0.0022 mm. 'These -conditions must cause that the infra-red energy of the sun is in the 5 tropics still more concentrated near the visible end of the spectrum than the figures givn above indicate. i

While the sun' is a body of a temperature of v probably more than 5500 degrees Celsius, the maximum of the radiation, of which, after transmission through the atmosphere, coincides on the average probably with the wavelength of maximum luminous efficiency, the light sources of film projectors, according to'a'temperature scarcely half as high, and according to the specific emissivity of tungsten radiate their maximum energy in the infra-red and at a wavelength about three times as long. The ceasing of the sun's, infra-red practically at 0.0022 mm., or in the tropics, where much water vapor though little real moisture is contained in the air, eventually already at 0.0015,

makes it possible to select appropriate heat-ab sorbers for this invention not only from a much larger number of substances, but also according to a different principle, according to which copper compounds on account of the proximity of their absorption band to the visible spectrum must be rated rather high. By the earlier re- .searches of Crookes, who by the use of black biotite mica ,isolated mainly the heat waves of a length of more than 0.0015 mm., only the particular suitability of filters for artificial radiation sources was tested and as is well known, no other eflicient absorber besides glass containing iron in the reduced state was found.

The properties required from'the window panes of the present invention are in the visible part of the spectrum perhaps still more different from those required from filters for artificial light sources, and particularly of kinematographic projectors, than in the infra-red.

According to the measurements of Allen (Phys.

' Rev., vol', 1900,p. 257) it is necessary for flashes of long waved light at low illuminations to follow each other much more rapidly than flashes of blue light if the eye is not to perce'ive iiicker. At that intensity of illumination at which the phenomenon had its maximum it was, necessary for flashes 01' yellow-green light to follow each other two an'd'a half times as rapidly as flashes of blue light. Since not only a shifting of the' hue particularly into the yellow-green, but also any increase of the intensity of illumination causes more flicker, it is clear that one condition for the desirable increase of brightness of the picture on the screen as rendered by the standard film of 16 pictures per second and by the present type of apparatus', is theavoiding of a high transmission specifically in the yellow-green.

Not only is the average brightness of the various parts of a moving'picture on the screen probably even below the average brightness of night interiors of roughly 0.1 millilambert, but the picture occupies in general also only the smaller part of the retina, so that the eye becomes adapted to a mean between the brightness of the picture and the general brightness of the other parts of the room, which is illuminated mainly by the reiiex from the screen. It has been shown already that for an active brightness of less than 0.1 millilambert the maximum sensibility oi the eye "length, and this 'also the intermediate articles oi Shirts beyond 0.000510 wave is one more reason. why any efllcient light filter or iilm prcjectors must be of a far more bluish color than if it had only the purpose oi suppressing the too abundant red and yellow rays of a tungsten filament.

I wish to include in the scope o! my invention I 'manufacture specifically prepared and compounded for the same, as for instance filters and colored materials the absorbers oi which are selected *for their high transmissivity in the yellow-green, though their transmissivity in the neighbouring parts oi the spectrum'may be still slightly higher, and which filters and-materials it is intended to use together with another filter for suppressing those too abundant rays. In ormer inventions concerning heat-absorbing windows the color was not considered at aii, so that an abscrption band situated right in the yellow-green would not have been a principal objectionior was even desired, as in the German Patent 160.358, where it is proposed to transmit only the blue and 'the orange-yellow rays in order to obtain a neutral white of relative coolness). In'the present inventionhowever, at least as much regard to a (non-monochromatic) transmissivity centered in the yellow-green is taken as to the regard to a high luminosity of the transmitted energy, so that the fulllment oi the letter regard is in general hampered by the i'ormer regard to a certain quality of the luminosity. The following example ,will make this ciear. If a composition of the transmitted energy corresponding to that of the threei'old filter recommended above as a standard were desired or demanded, and a certain hypothetical filter or absorber would transmit besides ioo percent oi? the 'yellow-green light also 100 percent oi light of 01100475 mm. wavelength, then according to the prescribed energy curve only 10 percent oi the transmitted energy of the letter wave-length with a luminosity value of only i percent can be counted as -iulflliing the requirements of the invention in this case, and the superfiuous 90 percent oi blue light have to be suppressed by another absorber or filter.

It must be presumed that the iron glass and the copperiron glass of thisinvention has been used already before as a colored glass for the "and low soda contents purpose of art and decoration in windows, or that the same or similar glasses have been tried also' as light filters in film proiectors. However, it has been shown hereinbeiore already that the absorption requirements (in There is further not only the diflerenoe in size between a window-pane and a filter' for a small film picture of at most one inch side length, which'diflerence has already a slight influence on the methods oi manufacture, but the high concentration of the hoat in the latter case on a small area makes the obtaining ot a low ocenicient of hoat expansion by the use oi high boren imperative. The high acidity, or borate forming power, o! boric acid, and the fact that iron borates as well as highly alkaline soda lime silicates are of yellow color, then shows again the necessity of employing di!- ierent methods for obtaining the adequate transmission and heat-absorption !or each ot the two difierent purposes.

Comparative tests as to m' samples oi glass or c! coatinzs the specific efliciency ot-equa lumi- .is provided on the mira-red) as well as the transmission requirements. (in the visible' nous transmission as iudged by the eye can be made by observing the quickness oi the rise within the first minute or other short time unit, or the total rise after a sufllcient time, oi! a thermometer with blackened bulb placed behind the samples. In connection with the present invention only the sun can be used as an energy source for experiments.

There exists a critical point `,beyond which in the case of this invention the selective absorption should never be augmented. When increasing the quantity of heat-absorbing substance per surface unit, the ratio between the lous effect on the one hand, and the energy which becomes stationary within the room on the other hand, will reach a point beyond which human comfort and efiiciency becomes smaller again, because the window glass by becoming hotter with growing absorption would hinder a decrease oi the temperature sufiicient to compensate for the increasing loss and the increasing color saturation of the light. The maximum color saturation of the panes of my invention is probably especially in those cases considerably lower than the average saturation of colored glass made for art and decoration, in which a heat-absorbing layer the outside of the pane, and n which the size of the windows has been restricted already Originally by reason od the costs of glass, or in order to limit the entrance of he The main argument usually brought orth against heat-absorbing windowsis that after a while the pane itself becomes warm and then communicates its heat in secondary form by radiation and air convection to the room. It can be shown, however, that in any case by lar the largerpart oi the heat is dispcsed ofi towards the outside, and that the room can be protected further from the heat of the pane by providing theheat-absorbing substanca in the form oi a Not only in the case of Figures 3 and 4,111 which a second sheet of ordinary glass is placed behind the absorbing pane, but also in the case of two penes .of ordinary glass, as often used' in cold climates, it is valuable to know that the airspace between the two penes should not be made too wide, as it is usually done. As the result of various researches (Smithsonian Physical Tables, tables 286 and 287) it was found that at ordinary temperature differences only in spaces up to iairly exactly one half o! one inch practically no air circulation at all takes place, and that the heat is transierred in this case only by conduction in quantities proportional to the width of the space. In vertical spaces of greater width than one half oi one inch the viscosity of the air is not sufllcient to hinder the establishment ora current which conveys, however, the less heat from one side to the other the higher the space, because its swiftness does not increase as fast as the height of the space, probably on account o! Mo eddies where the other square centimeter per hour per each 'degree celsius oi' the temperature difference oi' the two sui-faces forming the space have been found, after allowing for radiation, for the following most eifectiv'e widths:

0.250 calories at 1.4 cm. width in a space cm. high, v 0.167 calories at 1.6 cm. width in a space 20 cm.

high w 0.112 calories at 2.5 cm. width in a space 60 cm.

high.

There are reasons to assume that in a space of slightly `more than one half inch width the up and down current has the form oi a very thin sheet fiowing over the surface of the warm and the cold pane, and that by the collision of scattered air between the two currents always a core of prectically still air of the fairly constant thicknes of one half inch remains, no matter how wide the space. consequently I believe that the actual conditions can be well represented by the following rule according to which the most eective width of 'the space between two window penes is for all practical sizes equal to one half of one inch plus one fiftieth part of the height of the parties.`

The superiority oi' yellow-green eye protective glasses for augmenting visual acuity and for diminishing eye fatigue in bright sunlight has been ascertained by M. Luckiesh (Color and its Application, p. 154.1) beyond doubt. Yellowgreen eye glasses are not necessarily infra-red absorbing. Occasionally one and the same author advocating eye glasses of this color can be found to deny the feasibility or usefulness of heatabsorbing windows. In fact, however, it is only a paradox that in an effective heat-absorbing window just the hottest part oi the spectrum has 'to be transmitted. y

There exist& ,no natural standard of color at all. If only that'trifling quantity of blue and red rays above a certain threshold value for incitlng thesensation of color of the objects in a room is bontain'ed in the light, the complex of color with respect to the light itself is excluded in the mind; and in the case of .green light only the conceit otthat coolness arises of which it is'notonly suggestive, but which -mostly is its real and Physical property.. `The light in a room equipped with heat-absorbing windows of the proper color is ticalwith that under a green bojven wlich from hereditary reasons .considered as the 'most natural human sojon n since the recent time when the 'ered nearl'y everywhere with and layer-;3.1 and; representing intermediate articleeof. manufacture tor making the same,

ior electivelyitransmitting speciflcally the radibody of &temperature oi' morethan.

ation o! a WOOT-centi rade, characterized by containing 'a coloring matter ofnon-colloidal natureobtained 'non reducin'g conditions, .which'matter is of the elements iro a body of a temperature of more than 5000 centi- 'translucent sheet:

other constituents of the transparent material strongly an absorption band having its influence mainly in the red and the near-infrared of wavelengths shorter than 0.0015 millineter, and for' having under the same conditions a relatively high transmissivity in that central part of the spectrum of wavelengths longer than 0.00051 millimeter, for which-no single complementary hue exists in the spectrum, and which coincides with the range of the Purkinje eifect for illuminations of daylight inten'sities.

2. Transparent and highly translucent sheets and layers, and materials representing intermediate articles of manufacture speciflcally prepared and compounded for making the same, for the purpose oi' selectively transmitting the'radiation of a body ot a temperature of more than 5000 centigrade, characterized by containing a transparent compound, obtained under non-reducing conditions, of a. chemicai element with an atomic weight between 55 and 64-, which compound is selected for strongly exhibiting the absorption band of the respective element in the near infrared and for having a high transmissivity in thegreenish yellow and yellowish green region of 0 the spectrum, and by containing as aiurther addition a substance for subduing the transmission on the shortwaved side of this spectral region.

3. Transparent and anslucent sheets and layers, and materials speciflcally prepared and compounded for making the same, containing several nickel, cobalt,`copper in the form of transparent compounds selected for exhibiting the typical absorption band of the respective elements in the near intro-red and for having a relatively high transmissivity in the greenish yellow andyellowish green region arthe specl trum unto 0.00051 mm. wavelength, for the purpose of selectively transmitting speciflcally the radiation of a body of a temperature of .more than 5000 centigrade, as set forth. 1

4. A colored glass for selectively transmitting theradiation of a body of a temperature o! more 'than 5000 centigrade, characterized by containing the silicates of ironand copper, obtained under oxidizin'g conditions, all the constituents ot the glass being selected as 'to their nature and. quantity for producing by their coaction strongly' the typical absorption bands of iron and copper silicates and for having a high transmissivity in the yellow-green region oi' the spectrum of ,wavelengths longer than 0.00051 millimeter;

5. A colored glass' made speciflcally i'or the purpose of selectively transmitting the radiation of grade, as set` forth, characterized by containing 'the oxide of 'at least one of the elements iron and 'copper together with the oxide of at least one oi' the elements chromium and vanadium, all the constituents ofthe glass being selected as to their nature, 'and quantity .so as to produce by their coaction strongly the-characteristic absorption bands of the proper silicates of the respective ele'- mente, and toexhibit the highest transmission in the yellow-green of spectrum at a wavelength noj longerthan 0.00051 mlllimeter -t y ;wnumnrmxn 

